VANCOUVER — A Vancouver courtroom erupted in bittersweet cheers on Friday as a convicted sex offender who fed two teenage girls booze and drugs and then did nothing to help them as they slowly died was found guilty of criminal negligence causing their deaths.

B.C. Supreme Court Justice Bruce Butler also found Martin Tremblay guilty of one count of obstruction of justice related to the March 2010 deaths of 17-year-old Martha Jackson and 16-year-old Kayla Lalonde.

Butler said the girls had already been drinking in a Burnaby park when Tremblay — a drug dealer referred to by Lalonde as her “street dad” — invited them to his house to continue the party.

There, he gave them more alcohol and powdered methadone which they snorted off his kitchen table.

When Lalonde became ill that evening, Tremblay and a female acquaintance loaded her into his car and drove her to meet a drug-dealing colleague in Burnaby. There, Tremblay dumped her unconscious but still breathing onto the street.

“He should have sought immediate medical attention for her,” Butler said. “If he had done so, there’s a substantial likelihood she would not have died.”

Back in Richmond, Tremblay noticed that evening that Jackson was non-responsive but he didn’t seek medical help for her either.

“Instead of monitoring the situation, he took advantage of them,” Butler said, referring to Jackson and another teen.

Tremblay, who in 2003 pleaded guilty to five counts of sexual assault, videotaped himself fondling the two girls.

His actions showed “wanton and reckless disregard for their well-being,” Butler said.

Dozens of family members and friends of the girls came to hear the verdict, leading sheriffs to request a larger courtroom.

As Tremblay was led into the prisoner’s box Lalonde’s aunt stood up holding a poster with a picture of the pretty brown-eyed teenager with the message: In loving memory of Kayla Lalonde.

Both girls were aboriginal, and an elder held an eagle feather while others wiped away tears as the verdict was read out.

Outside the court, Lalonde’s mother, Angela Lalonde, said she felt justice had been done for her daughter.

“No matter what happens it won’t bring back our daughter,” said Grant Petrygan, her husband.

Kelvin Bee, Kayla Lalonde’s great-uncle, expressed relief that Tremblay will be off the street.

“The family is happy that the judge came through ... and not just for our family alone, but for the young women that the predators are out there for,” Bee told reporters outside the court.

“Our family still carries the grief. It’s just like reburying a loved one again.”

Jackson’s mother cried while the judge delivered the verdict and left without comment.

Tremblay, who faces trial in May on seven other charges related to similar sexual assaults on teenaged girls, could be jailed indefinitely if a dangerous offender designation was imposed.

“The Crown is considering sentencing options,” said Neil MacKenzie, spokesman for the criminal justice branch. “Certainly a dangerous offender application is within the range the Crown will be considering, but a final decision has not been made on that as yet.”

Lalonde’s family wants him declared a dangerous offender, which could mean an indefinite sentence. Details of the sex assaults committed the night Lalonde and Jackson died “sent us for a loop,” Petrygan said.

“I was shaking. I just about passed out; Angie was crying,” he said. “She was already passed out and dying as he was laying his hands on her. That’s sick. That’s a dangerous offender.”

Tremblay will return to court March 6 to set a date for his sentencing.